1. Field of the Invention
The subject invention is generally related to optical microphones and is specifically directed to a microphone having an optic element mounted for movement with the vibrating membrane of the microphone.
2. Description of the Prior Art
Conventional microphones, electro-acoustical transducers, are frequently used in electrically hostile environments. The output of the microphone is an electrical signal which can be distorted or interrupted by any electromagnetic interference, electromagnetic pulse or radio frequency interference. Since this output can be as low as several microvolts for some microphones subject to quiet or conversational level speech, the signal must be amplified near the source or the signal carrying wire must be adequately shielded to minimize distortion or interruption of the signal. These precautions are not always adequate, yet greatly increase the cost of the transducer system and add to the difficulty of manufacture and assembly.
More recently, fiber optic microphones have been developed, utilizing a fiber optic cable to carry the signal from the transducer to the amplifier. Fiber optic microphones are not dependent on electrical signals for voice transmission and provide for an electro-acoustical transducer with a minimum of distortion or interruption caused by electromagnetic interference, electromagnetic pulse or radio frequency interference. The light signal carried by the fiber optic cable is unaffected by electric or magnetic fields. It can be transmitted long distances without attenuation of the signal and without the need of added shielding. An example of an optical microphone is disclosed in Japanese Patent Application No. 0,041,055 by T. Nishida, filed in Japan on Sep. 18, 1978. The Nishida patent discloses a non-inductive microphone utilizing a vibrating membrane, a flexible fiber optic cable and a fixed lens mounted between the cable and the membrane.
U.S. Pat. No. 4,479,625 entitled "LASER MICROPHONE", issued to R. Muscatell on Oct. 23, 1984 discloses a laser microphone wherein two aligned beams of laser light are modulated by vibrations of a low mass reflecting body. The lenses are in a fixed position.
U.S. Pat. No. 3,622,791 entitled "MICROPHONE CIRCUIT FOR DIRECT CONVERSION OF SOUND SIGNALS INTO PULSE MODULATED ELECTRIC SIGNALS" issued to P. Bernard on Nov. 23, 1971 discloses a microphone for converting sound vibrations into electrical pulses by using a small mirror connected to the diaphragm. The oscillation of the mirror under the influence of speech will modify the optical path of directed light beams.
U.S. Pat. No. 4,640,382 entitled "ACOUSTIC FRICTIONAL RESISTANCE CONSTRUCTION AND METHOD OF PRODUCING AN ACOUSTIC FRICTIONAL RESISTANCE USING A LASER" issued to H. Hartmann on Feb. 3, 1987 discloses an electro-acoustic transducer which utilizes a laser to produce a light beam. The beam is reflected off of a mirror and then focused by a fixed lens.